Monolithic Microwave Integrated circuit (MMIC) chips operating at cryogenic temperatures have superconducting circuits that need to be thermally managed. One primary method is to remove the heat from the superconducting circuits down towards the substrate. Ground planes made of metal mesh material in lower level layers in the MMIC bring the entire layers to a thermal equilibrium. Because of this, portions of devices on the MMIC that need to be maintained at lower temperatures end up becoming exposed to higher temperature components.
At the cryogenic conditions, heat load, cooling resources, temperature, and circuit complexity are strongly tied to each other. The savings of one unit of power dissipation is magnified by multiple orders of magnitude when lifting from cryogenic temperatures to room temperature. As cryogenic chips become more and more complex, a greater number and greater variation of devices are populating the MMICs. Each of these devices can have different operational temperature requirements.
A typical cryogenic MMIC consists of a silicon substrate topped with alternating layers of electrically conducting material and dielectric. Multiple device types can exist in the MMIC. As an example, a MMIC may have three different device types that need to run at three different operating temperatures: The first device needs to operate below 500 mK (milliKelvin), the second device needs to operate below 1 K (Kelvin), and the third device needs to operate below 4 K. With a single ground plane, the entire mesh layer will be at a near uniform temperature. This is due to the ability of the electrically conducting material to transport (spread) the heat laterally in the X and Y directions. Therefore, if all devices are connected to this ground plane, all will have to be maintained to the most stringent operating requirement, for example, 500 mK. That is since the third device only needs to be kept at 4 K, but is instead maintained at 0.5 K, then 8 times the cooling resources are needed to manage this sector of the MMIC.